1. Field of the Invention
The invention concerns a radial cage for cylindrical rolling elements according to the generic features of claim 1, and it can be implemented to particular advantage in the cages of needle roller and cage assemblies intended for applications with very high rotational speeds.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Conventional needle roller and cage assemblies are single-row or double-row constructional units that consist of just a radial cage and a plurality of needle rollers, and are characterized primarily by a lowest possible overall radial height relative to the diameter of the needle rollers. Moreover, needle roller and cage assemblies produce bearings with high load capacity and high runout accuracy, which can also accommodate high centrifugal and acceleration forces. A generic needle roller and cage assembly of this type is known from EP 1,262,256 A1, for example, and consists essentially of two side rings that define the axial length of the radial cage and of a plurality of profiled axial crossbars that connect these side rings to one another at their outer diameters, with a plurality of pockets for accommodating the needle rollers formed between the crossbars. In this design, the axial crossbars are each composed of two axially straight side sections connected to the side rings, of two intermediate sections extending at an angle to the longitudinal axis of the cage, and of an axial center section, so that the needle rollers are held in the radial cage by the bottom edges of the side sections designed with angled stop faces and by the top edges of the center section of each axial crossbar.
Since such needle roller and cage assemblies generally are suitable for high rotational speeds for design reasons, they are used primarily for planetary gear bearings in the planetary gear trains of automatic transmissions in motor vehicles, and for connecting rod bearings and small end bearings of connecting rods in internal combustion engines. However, in newer generations of automatic transmissions, where the input speeds on the engine side are ever more frequently being stepped up to higher speeds, and the gear ratio speeds are often beyond 10,000 RPM, it has been shown that the increased loads resulting from the high rotational speeds have led to increased crossbar breakage in the vicinity of the straight side sections of the axial crossbars of the radial cage, and thus to failure of the needle roller and cage assemblies. The cause of this was identified as the fact that the radial cage is so strongly loaded by the intermittently acting, extremely high centrifugal forces that its axial crossbars bend outward due to their net mass, thereby causing braking of the needle rollers, and, under some circumstances, even blocking of the needle rollers. In combination with the support forces of the needle rollers on the side sections, this can then lead to the said crossbar breakage in the vicinity of the straight side sections of the axial crossbars of the radial cage, which then resulted in blocking of the needle roller and cage assembly and of the supported planetary gear, and thus in malfunctions of the entire automatic transmission.
To avoid these crossbar breakages, DE 10 2008 026,562 A1 thus proposed designing the axial center sections of each of the axial crossbars of the radial cage to have, in their center regions, an additional subsection extending to the dimension of the outer diameter of the side rings. This measure was intended to achieve the result that the center sections of the axial crossbars are supported on the inside circumferential surface of the bearing seat under high centrifugal loading, thereby reducing bending of the axial crossbars to a minimum.
However, in practice, it has been shown that although it was possible to reduce the frequency of crossbar breakage at the radial cages with such a measure, this type of crossbar breakage nevertheless continues to occur sporadically. Further investigations in this regard have shown that the cause thereof can be found in the particular pocket geometry in conjunction with the size of the stop faces for the needle rollers on the axial crossbars impressed on the bottom edges of the straight side sections. Since these side sections, like the side rings and the other sections of the axial crossbars of the radial cage, are made with a uniform material thickness corresponding to the thickness of the starting material, the angled stop faces do not have the height necessary for full-area contact with the needle rollers, but instead the needle rollers run against only the bottommost edge of the stop faces on the side sections of the axial crossbars.
As a result, increased wear occurs both at the ends of the needle rollers and at the side sections of the axial crossbars of the radial cage, which causes the transverse forces acting during bearing operation to be further intensified and to be transmitted to the side sections of the axial crossbars by the simultaneous contact of the needle rollers with the center sections of the axial crossbars via the leverage of the needle rollers that is now possible. As a result, further advancing wear of the side sections of the axial crossbars then occurs, ultimately ending in the said crossbar breakage of the radial cage with the resultant consequential damage.